In essence a computer program is a series of instructions performed by a computer. Quite often it makes sense to do several actions at the same time, in parallel. In a thread you can put the sequence of instructions that belongs to one of the parallel actions.

Threads are parallel lines of execution in an application.

A multi-processor computer may actually do these actions simultaneously, a single processor computer will divide its attention chronologically over the threads. In any case, the java code is the simple: it tells only which threads are running. The JVM takes care of the time sharing among the threads.

In the following applet, we visualize threads. The Ball class is a subclass of the class Thread, so each ball represents a thread. Clicking on the ball, will stop the thread and will make the ball disappear.

Examples

Typical examples of programs that use threads are:

• An applet containing an animation. A thread will ensure the sequential display of the images. Meanwhile, parallel actions remain possible: you still may press a button or use scroll bars. Actually, the event loop, that keeps track of the GUI-events, is another thread that is started automatically by the applet.
  
• A server application handling a variable number of clients. A separate thread will be started for each new client. You do not have to worry about how the threads share the cpu-time.